Recently, severe restrictions have been placed on the use of chlorofluorocarbons (CFCs) due to association of CFCs with the destruction of stratospheric ozone. In addition, CFCs have been labeled as environmentally unsafe in many countries worldwide. As a result, proposed alternative substances which can be substituted for CFCs in various CFC applications have been and are being developed. Among them are several new proposed hydrofluorocarbons (HFCs) such as the leading proposed substitute, HFC-134a and related compounds. These materials are already being sold as substitutes for CFC as refrigeration fluids. These replacement materials, while not ozone-depleting, contribute to the greenhouse effect. As a result, their use and escape into the atmosphere is the subject of the EPA's Significant New Alternatives Program, which limits the use of fluorinated compounds as alternatives for ozone-depleting chemicals. A. Lucas, "Report Airs Concerns About Fluorinated Compounds," Chemical Week, Feb. 8, 1995, p. 12.
The HFC replacement fluids are generally not as efficient as CFCs and require new types of lubricants for effective operation, thereby necessitating the redesigning of compressive-evaporative refrigeration and other systems using the HFCs. These new lubricants are sensitive to hydration, resulting in a tendency to absorb moisture from the air and moisture which has entered a charged refrigeration or other system through openings in the system such as from cracks and leaks. If water enters a refrigeration system having an HFC fluid, the water will degrade the lubricant promoting mechanical failure.
The newer working fluid refrigerants exhibit different solubilities than the CFCs, and are not miscible with well known lubricants in CFC systems such as conventional naphthenic mineral oil. Therefore, the new lubricants, including polyalkylene glycols (PAGs) and polyol esters (POES), have been developed. These lubricants are designed for the ozone-safe HFC-134a and related refrigerants.
Water in a system using these lubricants causes hydrolysis of the lubricating esters in a chemical reversion process. Esters undergoing acid-induced hydrolysis revert to their components with formation of alcohols and carboxylic acids. The lubricating properties of the esters degrade rapidly. With the resultant loss of lubrication, the compressive units experience excessive wear, and ultimately, catastrophic failure as the moving parts seize. B. D. Greig, "Formulated Polyol Ester Lubricants For Use With HFC 134a," Proceedings of the International CFC and Halon Alternatives Conference, (September, 1992), pp. 135-145. In U.S. Pat. No. 5,202,044, Hagihara et al. disclose that water severely affects performance of polyether based lubricants. These materials are hygroscopic and may contain up to 1500 ppm water. The moisture degrades their thermal stability in the presence of HFC-134a and causes organic materials such as PET films to be hydrolyzed.
The presence of water in refrigeration and other cooling, heating and ventilation systems has long been recognized as a problem and as having severe deleterious effects on such systems. The ubiquitous presence of moisture in the environment makes it extremely difficult to remove or eliminate moisture from such systems. Water is adsorbed on the surfaces of parts during manufacture and assembly including components such as evaporators, condensers and connecting tubing. It is present in refrigerants and other additives in varying amounts, is contained in lubricants and is introduced during charging or refilling of the systems. Water may also enter such systems through leaks around fittings and connectors, through hoses and cracks in metal that occur during operation and through improperly made connections.
These leaks also allow refrigerants or other working fluids to escape into the atmosphere, contaminating the environment and decreasing the efficiency and cooling capacity of the unit. If large amounts of cooling working fluids such as refrigerants escape, the system may overheat and the service life of the unit will thereby be shortened. Further, the unit may suffer mechanical failure from the loss of the working fluid. In general, leaks in heating and cooling systems also decrease the heat transfer efficiency of these systems.
Water in all types of compressive-evaporative systems decreases the system efficiency as a result of water's high heat of vaporization and high heat capacity. The high heat of fusion of water decreases the efficiency of a compressive-evaporative system by giving off heat in evaporation cycles as the water freezes. The resulting ice crystals can also block orifices in expansion valves and cause such systems to malfunction.
Water present in systems having metal, metal oxide or metal hydroxide surfaces also promotes surface oxidation. Such oxidation occurs within a compressive-evaporative system within the condenser, evaporator and connected metal tubing. These metal surfaces are coated with oxides and hydroxides of the composite metal due to the presence of water. The thickness of such a coating is dependent on the age and service life of the unit. The coatings are formed by oxidation of the metal caused by oxygen in the system as well as moisture which may also be present in the system. Formation is accelerated by the presence of acids. Even newly fabricated metal surfaces react with oxygen present in the atmosphere before unit assembly to form thin oxide layers on the metal surfaces. These thin oxide layers are termed "native" oxide layers. Air forms an oxide film 50 .ANG. thick on aluminum which increases with the presence of water. Metals Handbook, Desk Edition, American Society for Metals (1991), p. 6.64.
All oxidated surfaces have high degrees of polarity, and, consequently have high surface energies. These high surface energies readily attract moisture and hold it through electrostatic interactions such as hydrogen bonding. This molecular water layer can exist to varying degrees even before assembly of refrigeration systems. During operation, more water may bond to the metal oxide surfaces.
Lubricating oil may also occlude these high energy surfaces through electrostatic interactions. The oil forms a film of lubricating oil on the surfaces which acts as an insulator, hindering efficient heat transfer. Lubricating oil also accumulates in low areas of a refrigeration system, decreasing the lubricant available to the compressor. Accumulated oil may cover accumulated water that has collected in poor return areas of a refrigeration or other system due to the lower specific gravity of the oil. The combined layers of oxide, water and oil decrease energy transfer and reduce operating efficiency. These oxide layers continue to increase in thickness as long as water is present, and as a result, they continue to decrease the unit efficiency. This phenomenon has been reported recently by Komatsuzaki and Izuka, "Ester Oils As Lubricant For HFC-134a Refrigerator In Domestic Appliance," Proceedings of the International CFC and Halon Alternatives Conference, (September, 1992), Washington, D.C., p. 189.
The use of CFCs and HCFCs, while their use is now restricted, has been noted to reduce wear on compressors. This has been related to the chlorine content of these working fluids. The working fluids decompose to form active chlorine compounds which react with metal surfaces within the systems to form protective metal chloride boundary layers. These layers have a positive anti-wear affect. K. E. Davis and J. N. Vinci, "Effect of Additives In Synthetic Ester Lubricants Used With HFC-134a Refrigerant," Proceedings of the International CFC and Halon Alternatives Conference, (September, 1992), Washington, D.C., p. 125. The active chlorine compounds produced can also combine with water present in the system to produce strong acids. These acids can be transported to evaporators and condensers where they corrosively attack metals and have the potential to cause leakage to the environment.
Copper plating, which produces wear, occurs in CFC, HCFC and HFC systems and is also related to the presence of water in the system. This phenomenon would be aggravated by polyol ester lubricants which can absorb up to 1500 ppm water from the atmosphere.
One current method for the removal of moisture from refrigeration and other systems includes providing a dryer unit to the system. The dryer units in refrigeration systems are typically positioned in the liquid, high-pressure refrigerant area at the outlet of the condenser units. These dryer-strainer units contain desiccants such as silica gel which attract water molecules to their surfaces. The water is held to a desiccant's surface by hydrogen bonding of the polar water molecules to the polar desiccant surface. This is an equilibrium phenomenon in which water molecules can be transferred back to the refrigerant. The desiccants employed have a high capacity for water entrapment, but a low affinity for the actual water molecules as indicated by their failure to completely remove water from the system and their slow action in achieving the absorption.
Alternatives to silica gel desiccants include zeolite systems in which water molecules are entrapped within pores in a zeolite. Zeolite systems are more efficient than silica gel desiccants in removing water. However, zeolites have a higher affinity for water, a low capacity for water entrapment, and are even slower to dehydrate than silica gel desiccants.
In refrigeration or other systems using dryers, as described above, water remains in the system in some form, for example, adsorbed on the desiccant surface, trapped within zeolites, or circulating within the system itself. It is not transformed to another species. Early attempts to chemically remove water are reported in U.S. Pat. No. 2,185,332 of Crampton.
Crampton describes adding sodium alkoxides to refrigeration systems which react with water to produce alcohol and sodium hydroxide, a very strong inorganic base that is insoluble in all compressive refrigeration system fluid components. The reaction products are extremely corrosive and would be expected to chemically react with system refrigerants causing their chemical breakdown or with metal surfaces within the system causing component deterioration. The acid neutralizing properties claimed by Crampton would produce sodium chloride, another insoluble product. Formation of such insoluble particles has a detrimental effect on operating refrigeration systems by rapidly increasing wear of moving mechanical parts and by blocking orifices required for efficient operation. Sodium alkoxides are very strong organic bases and would be expected to react with refrigerants and other halogen-containing working fluids by hydrogen-halogen extraction causing working fluid decomposition.
More recent attempts to remove moisture and seal systems are described in U.S. Pat. Nos. 4,304,805, 4,331,722, 4,379,067, 4,442,015 and 4,508,631 of Packo et al. which teaches the use of silicon-containing compounds including certain mercaptosilanes, acyloxysilanes, aminosilanes, and alkoxysilanes in conjunction with acetic anhydride or aminosilanes.
The organotrialkoxysilane compositions of Packo et al. produce insoluble organosilsesquioxanes on reaction with water. Introduction of nitrogen-containing species, such as the aminosilanes, produces insoluble ammonium or amine salts in the presence of system acids, and generates toxic ammonia within the system. Ammonia and amines in systems employing fluorocarbon-based working fluids may promote undesirable chemical reactions leading to working fluid decomposition. Furthermore, the presence of salts within a system employing electrical connections is precluded for safety reasons. Incorporation of amines or ammonia-generating compounds renders indicating devices on cooling and refrigeration systems (such as "Dry Eye") inoperable by indicating a safe condition when one actually does not exist. Some moisture indicators turn blue in the presence of bases such as ammonia. This interaction provides a false reading as water may be present in the system even though the indicator exhibits a blue color. Ammonia or amines can also produce stress corrosion cracking of copper or brass, a substantial component of some refrigeration systems which use CFC, HCFC and HFC working fluids. Metals Handbook, Desk Edition, American Society for Metals (1991), p. 7.37.
Sydney has reported that lubrication is extremely important in all compressive-evaporative refrigeration systems. D. Sydney, "Why Compressors Fail Mechanically," Air Conditioning, Heating and Refrigeration News, .sctn..sctn.1-6 (1991). Fluorocarbon systems pump a finite amount of lubricant with the refrigerant working fluids. The refrigerant flowing back to the compressor is intended to carry that oil back to the compressor. Compressive-evaporative fluorocarbon refrigeration systems are designed and installed to perform this function by maintaining a low pressure drop to ensure proper oil return. Oil collecting in evaporators and coating the tubes causes a drop in evaporative pressure and insufficient oil return. The lack of oil returning to the compressor causes improper lubrication. Poor lubrication results in oil and compressor overheating as well as excessive wear. High oil retention in the evaporators and condensers leads to compressor failure from the excessive wear and overheating or to motor burnout.
The thermal stability of dehydrating compositions is important in compressive-evaporative refrigeration and other heating and cooling systems due to the harsh and extreme conditions of those systems. Operating systems employing compression-evaporation cycles have pressure and temperature extremes that may promote component degradation. A typical HCFC-22 system will have pressures of about 200 to 300 psi and temperatures of about 200.degree. to about 225.degree. F. Temperatures within compressors can typically approach 275.degree. F. Units not operating properly, for example those having restricted valves or insufficient working fluid or lubricant return, may exceed operating temperatures of 300.degree. F. If temperatures of about 350.degree. F. occur in compressors, such conditions may cause lubricants to "coke," i.e., to turn to carbon, such that valves may become carbon-coated and particles may be generated which tend to clog the strainer/dryer resulting in reduced working fluid flow. Decomposition of the refrigerant or other working fluid at these operating temperatures produces the elemental components of the working fluid including halogens such as chlorine and acids such as hydrochloric acid. A need exists, therefore for a dehydration composition which is able to maintain its integrity and reactivity without degradation or functional change under these harsh operating conditions.
New regulations eliminating manufacture of CFC-12 and HCFC-22 in the year 2002 based on the ability of these refrigerant working fluids to deplete ozone make it imperative that these materials not be released from systems employing them. In addition, the new alternatives such as HFC-134a or HFC-152a should not be released due to their nature as greenhouse gases which contribute to global warming. The containment of these gases within operating units will exhibit beneficial effects to the environment and reduce costs related to securing these materials and converting to further alternative working fluids. The containment of these materials can best be addressed by the environmental isolation of the system using them. A need in the art exists for a method for isolating these systems which takes account of the working fluids, lubricants and methods of operation of the current systems.
In sum, a need in the art exists for a method for sealing leaks in refrigeration, air conditioning, heating, ventilation and related systems and for the complete dehydration of the systems. Complete dehydration is desirable for ensuring proper operation without formation of insoluble particles, gels or varnishes that result from reaction of sealants and/or other additives with contained system moisture.